The invention pertains to welding and separating devices for stacked plastic films. This invention also relates to a method for welding stacked plastic films. More particularly, the devices include a heated welding bar positioned on one side of the film. The bar carries a welding/separating strip. A counterpressure element is positioned on the other side of the film. The welding/separating strip, on its side facing the films, has about in the center thereof, a separating knife which is preferably wedge-shaped in cross section. On either side of and at a distance from the knife are welding strips which are preferably shorter than the separating knife. The counterpressure element consists of a springy, resilient material.
Many devices for welding plastic films are known in the form of heated welding strips or welding bars, which press the films against a resilient, flat backing to bring about the weld. If it is desired, as frequently occurs in packaging technology, to separate welded film sections, it is conventional practice to use two parallel and closely adjacent welding strips simultaneously to produce two welding seams, between which the separation then occurs.
For separating, there are also various known alternatives. One of these is a heated separating wire which is pressed through the films. A fixed, heated knife and a moving, cutting knife are also known. Hold-downs for the film are usually employed when knives are used. It is also known, finally, that cutting and welding can be carried out with a single element, namely, a creasing strip or a heated round wire. In this case, it is necessary to be satisfied with a comparatively narrow weld seam and a direct transition from weld seam to separated edge.
In West German Offenlegungsschrift No. DE 28 01 201, published July 19, 1979, for example, one sees a simple welding and separating device appropriate for manual welding. It has a resistor welding strip and a movable pressure bar. The quality of the welding seam and the productivity of devices operating at this level of technology are comparatively low.
Various other types of welding and separating devices are known from British Patent Specification No. 1,273,245; West German Offenlegungschrift No. 31 41 898, published May 11, 1983; West German Patent No. DE 10 78 318, dated Sept. 22, 1960; and West German registered utility model No. 72 34 088, dated Sept. 15, 1972. The welding and separating devices discussed in these documents reveal varied forms for such devices used in industry, particularly in the packaging industry.
A problem occurring especially at high productivity rates in devices for the simultaneous production of a double seam with a separation occurring between them is the occasional complaints of defective welded seam quality. In particular, this takes the form of an excessive thinning of the material; and ruptures in the material can even occur in the area of the welded seam.
The present invention is based on the task of improving a welding and separating device of the type described above in such a way that the uniformity in quality of the weld seams is greatly improved. This task is solved, according to the invention, in that the longitudinal edge of each welding strip (seen in cross section) facing the counterpressure element is preferably rounded with a radius which is preferably equal to about half the thickness of the welding strip; and in that the surface of the longitudinal edge curved in this way, starting with the outer side of the welding strip, extends essentially in a continuous manner to its meeting point with the preferably lateral wedge surface of the separating knife bordering the welding strip on the inside; in that the line of intersection between the longitudinal curved surface and the wedge surface is about at the height of the center of the curvature of the longitudinal surface or deeper; and in that the counterpressure element (seen in cross section) has an outward-arching surface positioned so that the smallest distance of which to the welding/separating strip is approximately in the center longitudinal plane of the welding/separating strip.
In a preferred arrangement, support bars are positioned between the counterpressure element and the welding strip. As the element and strip move together, the support bars contact the element to deform it to a desired shape and desired degree of deformation (reduced resiliency) to reduce tension in the films as they are separated and welded.
Advantageous embodiments of these solutions are given in the subclaims.
By means of the agencies described, the desired goal is achieved in a manner which is as simple as it is convincing. A welding and separating device designed in accordance with the invention creates seams of uniformly high quality with unexpectedly high service lives for the counterpressure element and the welding/separating strip even at very high productivities, i.e., at a large number of welds per unit time. It has also been found that, in contrast to the state of the art, it is no longer necessary to match the cutting edge of the separating knife to a hard cutting surface on the surface of the counterpressure element. This cutting surface was a strip of fiberglass, which was glued to the flat surface of the known counterpressure element in the area of the separating knife. The elimination of this strip, which is now possible, has the additional advantage that the known, time-consuming alignment of the separating knife with respect to the narrow strip, which is attendant on tool replacement, is now completely eliminated because, with the counterpressure element having a rounded surface, a rough alignment is completely sufficient.
In addition, it has been found that the gap between the welding strips of the welding/separating bar can be reduced by comparison to the state of the art, and that a much smaller amount of film residue collects than has customarily been the case at the wedge surfaces of the separating knife. The reason for this appears to be that the device according to the invention leads to lower tensile stresses inside the film in the welding and separating area.
In calculating the dimensions of the counterpressure element and the gaps between the welding strips and the separating knife, the goal to be achieved is that the welding strips contact the counterpressure element when they are brought together at a point which is in or near the V-shaped recess in the surface of the counterpressure element brought about by the separating knife. This has the desirable result that a comparatively thin welding/separating strip can be used with the possibility of creating very narrow weld seams. Narrow weld seams make hardly any negative visual impression and, at a width of 1/2 mm, for example, are in practice just as strong as the seam produced by the usual means with a width of about 5 mm. Narrow weld seams also require a reduced cooling time, which makes it possible to increase productivity. Finally, a welding/separating strip of reduced thickness also means that the gap to be provided for it in the plane of conveyance of packaging machines incorporating the present invention can be correspondingly narrower, so that the movable coverings conventionally used there may frequently be eliminated.